Semiconductor devices are used in many electronic and other applications. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits.
One type of semiconductor device is a memory device, in which data is typically shored as a logical “1” or “0”. Memory devices may be static or dynamic. Dynamic memory devices need to be refreshed to “remember” the data, whereas static memory devices do not need to be refreshed to retain stored data.
One type of static memory device, also referred to as a non-volatile memory (NVM) device, is a floating gate device. Floating gate memory devices can be either erasable programmable read-only memory (EPROM) or electrically erasable programmable read-only memory (EEPROM). Both of these floating gate memories rely on charge stored in the floating gate (or a charge trap layer) by suitable application of a bias to the various terminals of the device. The charge may be stored by a number of mechanisms such as carrier tunneling and/or injection. The charge may be removed either electrically as in EEPROM or by an external source such as an ultra violet light. The present of this charge in the floating gate determines the state of the memory “1” or “0”. Flash EEPROM memories are so called due to their fast program and erase times (as in a lightning flash).
The floating gate devices may be stacked in large arrays to form memory cells such as a Flash memory cells. Based on the stacking or layout of the floating gate transistors, flash memories may comprise NOR, NAND or a AND memory architecture. As an example, most commercial memory cards such as memory sticks comprise NAND flash memory cells. Flash memories are amongst the most popular memories available in the market today. The popularity of flash memory arises partly due to its compatibility with existing CMOS process flows. Flash memory is simply a field effect transistor except it has a polysilicon floating gate (or silicon nitride charge trap layer) sandwiched between a tunnel oxide and an inter-poly oxide to form a charge storage layer.